This invention relates to bismuth-containing temperature-stable dielectric ceramics that are stable over a wide range of frequency, and thus suitable for NPO and microwave applications.
Some dielectric materials exhibit very stable dielectric properties from low frequency through the microwave frequency range. The basis of this class of frequency-stable compositions is the temperature compensating NPO materials described in U.S. application Ser. No. 318,698, filed Mar. 3, 1989 by Bardhan et al under the title CAPACITORS AND HIGH DIELECTRIC CONSTANT CERAMICS THEREFOR, wherein a material with a high dielectric constant, a low dissipation factor, and a small temperature coefficient of capacitance is described.
Though the vast majority of NPO materials is evaluated at a single frequency based upon the capacitance of the material, the effects of varying frequency with respect to each of the said factors are known to the art. In conventional ceramics, the dissipation factor is known to increase with increasing frequency, whereas the dielectric constant is known to decrease with increasing frequency.
Temperature-stable dielectric ceramics that remain stable over a wide frequency range are useful low K NPO ceramic capacitors that maintain low impedance in the integrated circuitry of high speed computers, microwave dielectrics for capacitors or resonators, and substrate materials for microwave circuits. Unlike the temperature compensating materials described in U.S. application Ser. No. 318,698, these materials are also evaluated in terms of the dielectric Q factor, the reciprocal of the dissipation factor, and the temperature coefficient of resonance frequency, .tau..sub.f. The temperature coefficient of resonant frequency is defined by the equation EQU .tau..sub.f -[.tau..sub..epsilon. /(2+.alpha.)]
where .tau..sub..epsilon. is the temperature coefficient of the dielectric constant and .alpha. is the linear thermal expansion coefficient. The physical constant, .tau..sub..epsilon., relates to the measurable parameter, TCC, the temperature coefficient of capacitance via the equation EQU .tau..sub..epsilon. =TCC-.alpha.
High Q factors and frequency stability are required for small TCC's, though a small TCC does not necessarily imply a high Q factor or frequency stability.
Perhaps the most common use of this class of frequency stable NPO materials having a high dielectric constant and a high Q factor at microwave frequency is that of dielectric resonators, devices that act as filters and stabilizers of oscillations in microwave circuits. In the 1960's titania was most often used as a dielectric resonator, exhibiting a dielectric constant of 100 and a Q factor of 10000 at microwave frequency. However, the temperature coefficient of resonance frequency, .about.+400 ppm/.degree.C., was too high for practical applications. Since then, a number of titaniabased, temperature-stable ceramics with high dielectric constants have been explored with relative successes, among which are the BaO-TiO.sub.2, ZrO.sub.2 -SnO.sub.2 -TiO.sub.2, and MgO-TiO.sub.2 systems, as can be readily seen in U.S. Pat. Nos. 4,753,906 and 4,665,041 discussed later in the prior art. More recently, these and a host of other materials have been adapted to such uses as automobile telephones and receivers for satellite broadcasting.
Dielectric resonators are often used in receivers for satellite broadcasting or SHF broadcasting on the ground. When a dielectric resonator is used at the high frequency bands common to broadcasting, such as 0.5-6 GHz, several shortcomings may occur: (1) since their specific dielectric constants are small, it has been impossible to sufficiently miniaturize the resonator; (2) the Q factor may become small (the dielectric loss becomes large); and (3) the nonlinearity of the change in resonance frequency accompanying changes in temperature makes it difficult to compensate for the temperature characteristics. Conversely, the desired properties of such dielectric materials are a dielectric constant large enough to satisfy the requirement of reduction in size, small dielectric losses at high frequencies, and the change of the resonance frequency with respect to temperature, i.e., the temperature dependence of the dielectric constant, should be small. In general, desired dielectric properties of dielectric properties targeted for resonators are: a high relative dielectric constant (30-40); a high Q factor (&gt;3000 at 10 GHz); and a low temperature coefficient of resonant frequency (0.+-.10 ppm/.degree.C.). More specific and stringent standards may be required, however, as indicated in European Patent 0,095,338, which discloses the need to improve the performance of the receiver by improving the dielectric properties of the materials used in producing the dielectric resonator; namely, a dielectric constant of 25 to 40, an unloaded Q factor of 10.sup.4 or more and a temperature coefficient of resonant frequency of within .+-.10 ppm/.degree.C. in a 10 GHz band and at about room temperature.
It is therefore an object of this invention to provide novel temperature-stable compositions for use in the development of, but not limited to, frequency stable ceramic capacitors.
It is another object of this invention to provide an NPO ceramic material with a dielectric constant that is stable from low frequency through the microwave frequency range.
It is yet another object of this invention to provide an NPO ceramic suitable for producing dielectric resonators.